Radio frequency receivers have been widely used in various electronic products such as AM and FM radios, television sets, and GPS (global positioning system) navigation devices. Typically there are multiple channels within the allocated spectrum. In order to receive the signal in a desired channel, the radio frequency input signal is usually mixed with a signal generated by a local oscillator (LO) to translate the incoming radio frequency signal to a lower-frequency signal suitable for further processing using cost effect components and/or for superior performance. The frequency translated signal may be a baseband signal, low-IF (intermediate frequency) or IF signal. The low frequency characteristic of the frequency translated signal makes itself ideal for digital signal processing at lower clock speed to conserve power consumption. In addition, the use of digital signal processing technique provides high flexibility for processing the underlying signal. Therefore, the radio receiver usually includes digital signal processing circuitry to perform the required receiving functions such as filtering, demodulation, and de-multiplexing (for FM stereo broadcast).
In order to receive a desired channel, the incoming radio frequency signal is mixed with a selected LO signal to translate the frequency of incoming radio frequency signal to a lower frequency signal. A bandpass or a low pass filter is applied to the mixed signal in order to filter out possible interfering signals. The proper mixer operation requires a desired LO signal to be generated responsive to the channel selection. The LO signal usually is generated by a tunable clock generation circuit which typically includes a voltage controlled oscillator coupled with phase locked loop circuitry. The LO frequency has to be substantially accurate to ensure proper system operation and good sound quality. Furthermore, system clocks for various parts of the radio receiver such as digital clock and sampling clocks for analog-to-digital converter (ADC) and digital-to-analog (DAC), also have to be substantially accurate to ensure proper system operation and high system performance. In order to meet the LO frequency and system clocks accuracy, a crystal is often used to provide the required accurate reference clock so that the LO signal and system clocks can be accurately generated using techniques such as phase-locked-loop to lock the LO frequency and the system clocks with the reference clock. Alternatively, a reference clock complying with the required frequency tolerance may also be used. The LO frequency may be locked to the external reference clock. The use of a crystal may increase the system cost. The use of an externally supplied reference clock will relieve the need for a crystal. However, an externally supplied reference clock may not be always available. For example, in a stand-alone radio receiver application, the external reference clock may not be available.
In light of the foregoing discussions, therefore it is desirable to provide systems and methods for clock generation that do not require a crystal or a reference clock. The clock generation should provide an accurate LO frequency for the receiver to tune to a desired channel and provide an accurate system clocks to the receiver for proper operation without the need of a crystal or a reference clock.